Virginia and UNC would really beef up Big Ten Lacrosse.
They would definitely overtake the ACC as the top dog at that point, leaving them with only Duke, Cuse and ND (which is of course still pretty good).
I'm sure this will be factored heavily into their decision. 
To expand on this since you are all on the edge of your seat with bated breath, there are only 8 teams to win more than one NC in the NCAA Tournament era (1971-present). So those would be the 8 "Helmets" of College Lacrosse. They are:
- Syracuse - 10 (14 overall)
- J Hopkins - 9 (36 overall)
- Virginia - 7 (8 overall)
- Princeton - 6 (14 overall)
- N Carolina - 5
- Maryland - 3 (10 overall)
- Cornell - 3 (4 overall)
- Duke - 3
So set aside the two Ivys, and you have Cuse and Hopkins as the two "Uber Helmets," plus the old guard ACC quartet of Maryland, Virginia, N Carolina and Duke as the run of the mill "Helmets."
So if the Big Ten poached UNC and Virginia, then they would have one Uber Helmet, and 3 more Helmets.
The ACC would have 1 Uber Helmet as well, but only 1 regular Helmet, each of which would be the "worst" in their respective category. Now they will tout that they have more "Helmets per capita" and they will argue that their non-helmet (Notre Dame) is by far the best team in the Midwest, and they would point out that they don't have any hilariously bad teams like the Wolverines... and while all this would be true, the reality would be that we picked them clean.
Virginia won the last two NCs, Fwiw.